Lignin, an aromatic polymer is a major constituent in e.g. wood, being the most abundant carbon source on Earth second only to cellulose. In recent years, with development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry.
Lignin, being a polyaromatic network has been extensively investigated as a suitable substitute for phenol during production of phenol-formaldehyde adhesives. These are used during manufacturing of structural wood products such as plywood, oriented strand board and fiberboard. During synthesis of such adhesives, phenol, partially replaced by lignin, is reacted with formaldehyde in the presence of either basic or acidic catalyst to form a highly cross-linked aromatic resins termed novolacs (when utilizing acidic catalysts) or resoles (when utilizing basic catalysts). Currently, only limited amounts of the phenol can be replaced by lignin due to the lower reactivity of lignin. Lignin is utilized as a powder at the time that it is incorporated into the resin formulation. Lignin can also be utilized in “liquid form” in an alkali solution in order to avoid lignin dust. If the moisture content of the powder lignin is relatively low (0-5%), it can be dusty and may create respiratory hazards. If the moisture content of the solids is relatively high (8-50%), it can be sticky or clumpy and difficult to transfer in reliable and quantitative manner.
The major problem concerning the use of lignin alkali solution is that the viscosity of the lignin solution becomes too high if the lignin content is increased to a certain level. In a resin for laminates, the solid content of the resins is typically higher than for example in plywood and the amount of water in the lignin solution makes it difficult to produce a resin with higher solid content.
U.S. Pat. No. 3,216,839 describes solubility properties of free lignin in a solution of essentially methanol or only a small amount of water. It is described that the solubility effects are only observed at a high concentration of methanol and in the absence of alkali.
For certain types of applications, such as in the preparation of laminates, there is thus a need to be able to increase the lignin content in the resins used.